"Are they dead? Did they attempt to flee the site after the attack like some other expatriates? Are they lost in the desert after taking a wrong turn?" said a spokesman for prime minister Abdemalek Sellal. "These are all questions we ask ourselves."

The Ain Amenas gas plant, jointly run by BP, Norway's Statoil and the Algerian state oil company, is deep in the Sahara, 800 miles from the coast, with few population centres nearby.

The desert in the area is flat rocky and featureless and while roasting hot in the summer, during the winter months the temperature drops to 3C at night, with average highs of 18C during the day.

An al Qaida-linked band of fighters attacked the vast natural gas complex on January 16, killing 37 hostages.